The present invention generally relates to optical image sensors, and more particularly to the integration of a microlens with an underlying photocell on a semiconductor chip as part of an optical image sensor.
The use of microlens arrays in imaging devices, such as cameras, is known in the art. U.S. Pat. No. 6,307,243 discloses a microlens array for use in a solid-state CMOS imager in which a plano-convex microlens and an underlying photodiode define one of many pixels of an array. The principal element of each microlens is fabricated from an organic material that is heat-treated to provide a convex upper surface and a planar lower surface.
U.S. Pat. No. 6,157,017 discloses a solid-state imaging device in which an array of microlenses is formed by reflowing a two-dimensional array of spaced photoresist regions. Each resulting microlens has a convex upper surface, a planar lower surface, and an index of refraction in a range between about 1.65 and 1.70. Red, green and blue filters are included, one under each microlens with a photodiode under each filter. Concave depressions are formed in a dispersion layer between adjacent microlenses to contribute to focusing of light towards the photodiodes.
U.S. Pat. No. 6,221,687 discloses a color CMOS image sensor having a matrix of pixels including microlenses, each microlens being located beneath a color filter layer and above a photodiode. Each microlens is formed by depositing a dielectric layer of a material such as silicon nitride, forming a lens-shaped photoresist portion on the dielectric layer, and then performing an anisotropic reactive ion etching process that copies the lens-like shape of the photoresist portion into the dielectric layer. The result is a dielectric layer having a planar lower surface and a microlens portion having a convex upper surface.
It would be desirable to provide a simplified process for forming a microlens in an optical image sensor while enhancing the signal-generating performance of the sensor.